Optical transmitters are provided in, for example, an optical communication system, and converts electrical signals to optical signals so as to transmit the thus converted optical signals. Conventional optical transmitters are described in, for example, Patent Documents 1 and 2.
Patent Document 1 describes an optical transmitter that includes a laser diode (LD) as an optical source; a transistor into which an ON/OFF signal of the LD (hereinafter, “LD ON/OFF signal”) is input and that controls a drive current of the LD accordingly; and an external modulator that modulates light output from the LD with an input signal. The circuit of such an optical transmitter is configured in such a manner that the collector of the transistor is connected to the cathode of the LD, a load resistor is provided between the emitter of the transistor and the ground (GND), and an LD ON/OFF signal is input to the base of the transistor. Thus, ON/OFF of the laser diode (LD) is controlled.
Patent Document 2 describes an optical transmitter that further includes a photodiode (PD) that detects an optical output from the LD; and a feedback control circuit such as a feedback automatic power control (APC) circuit connected to the PD, in addition to the configuration of the optical transmitter described in Patent Document 1. In such an optical transmitter, an LD drive current is controlled by using the feedback control circuit according to the detection results of the optical output from the LD detected by the PD, to thereby control the optical output from the LD to be made constant. In this optical transmitter also, the optical output can be turned ON or OFF according to the LD ON/OFF signal.
[Patent Document 1] Japanese Patent Application Laid-open No. H9-83050
[Patent Document 2] Japanese Patent Application Laid-open No. 2000-89178